The results are particularly important in developing countries, where the cost of providing three HPV shots per person makes the vaccine impractical to administer, the researchers said. About 85 percent of the world's cases of cervical cancer occur in developing countries, which lack effective cancer screening programs.

"We are very enthusiastic about these findings," said study researcher Aimée Kreimer, of the Division of Cancer Epidemiology and Genetics at the National Cancer Institute. "If they're confirmed by other studies, then providing this vaccine to girls and women around the world will be much cheaper and much easier. In the end, it could really reduce the amount of cervical cancer there is."

However, more research is needed to see if the results hold in other populations, and to determine if one or two doses of the HPV vaccine can be effective for more than four years, Kreimer said.

HPV vaccination
The study involved 7,466 women ages 18 to 25 who were randomly assigned to receive three doses of the Cervarix HPV vaccine or three doses of a control vaccine for hepatitis A. About 20 percent of participants did not end up receiving all three doses, usually because they became pregnant (the HPV vaccine is not given to pregnant women) or they were referred for a colposcopy (an exam in which a visual inspection of the cervix is performed, to see abnormal-appearing cells).

In all, 5,967 women received three vaccine doses (2,957 received the HPV vaccine; 3,010 received the hepatitis A vaccine), 802 received two doses (422 of the HPV; 380 of the hepatitis A), and 384 received one dose (196 of the HPV; 188 of the hepatitis A).

Samples from the subjects were tested for the DNA from HPV 16 and 18. The researchers looked for persistent HPV infections that lasted 10 or more months.

Statistically, there was no difference in the HPV vaccine's efficacy among the three groups that received different numbers of doses, the researchers said. The vaccine was found to be 80 percent effective among the women who received three doses, 84 percent effective for two doses and 100 percent effective for one dose.

Rates of women who are opting for preventive mastectomies, such as Angeline Jolie, have increased by an estimated 50 percent in recent years, experts say. But many doctors are puzzled because the operation doesn't carry a 100 percent guarantee, it's major surgery -- and women have other options, from a once-a-day pill to careful monitoring.

Is one dose good enough?
The Cervarix vaccine does not contain the complete human papillomavirus, only virus proteins. Vaccines made this way have historically needed more than one dose to be effective in the long term, Kreimer said.

"The vaccine, when given in three doses, elicits a very strong immune response," she said. Still, even the subdued immune response brought about by one dose might be adequate protection.

Cosette Marie Wheeler, of the department of obstetrics and gynecology at the University of New Mexico, noted that the study was not specifically intended to evaluate vaccine HPV vaccine efficacy with fewer than three doses.

Researchers need to conduct "additional larger studies that are specifically designed to evaluate the efficacy of one-, two-, and three-dose regimens in young adolescent girls," Wheeler wrote in an editorial accompanying the study, which was published Sept. 9 in the Journal of the National Cancer Institute.

"The age-old adage of 'Less is more' may apply to HPV vaccination, and if so, the report … represents an important step on the road to more-effective and sustainable cervical cancer prevention programs," Wheeler said.